This invention relates to a system for the low-temperature fractionation of air by two-stage rectification in a high-pressure column and a low-pressure column wherein nitrogen is withdrawn in the gaseous phase from the upper part of the high-pressure column, warmed, compressed, cooled, liquefied, and then recycled into the upper part of the high-pressure column.
A process for air fractionation is known from British Pat. No. 975,729 wherein gaseous nitrogen is evolved in the high-pressure column of a two-stage rectification unit. This gaseous nitrogen is compressed at a temperature of below -50.degree. C., cooled by indirect heat exchange with an external refrigerant, and then recycled via a pressure reducing valve into the high-pressure column.
This conventional process is employed if a large quantity of a liquid external refrigerant is readily available, and it is desired that this external liquid refrigerant be converted into the gaseous phase. As compared with conventional air fractionation processes, the steps of compression of the gaseous nitrogen withdrawn from the high-pressure stage and the subsequent cooling of the nitrogen result in a saving in energy. However, a significant disadvantage of this method is the fact that the external refrigerant is utilized merely for the production of liquid nitrogen, rather than also for lowering energy consumption during the fractionation process.